Antenna system for multiband radio receivers



March 23, 1937, D. P. EARNSHAW ANTENNA SYSTEM FOR MULTIBAND RADIORECEIVERS Filed Dec. 2l, 1935 Patented Mar. 23, 1937 UNITED STATESPATENT OFFICE ANTENNA SYSTEM FOR MULTIBAND RADIO RECEIVERS ApplicationDecember 21, 1935, Serial No. 55,647

In Great Britain December 24, 1934 6 Claims.

This invention relates to antennae or energy collectors forradio-receiving systems and the like, and particularly to antennae formulti-band receiving systems. One object of the invention is to improvethe quality of radio reception by increasing the efficiency of theantenna as a signalcollecting device and by reducing the eiect on thereceiving system of electrical disturbances in the vicinity of theantenna lead-in conductor. 'I'his 10 object is accomplished asfollows:-First, the device provided by the invention automaticallychanges the length of the antenna from an electrical point of view sothat the proper length of antenna is provided for` each frequency bandwithout changing the antenna physically. Second, a lead-in line isprovided between the antenna and the receiver for the transfer ofsignals to the receiver; which line is designed so as to effect minimumattenuation of signal strength,

minimum pick-up of local interference, and maximum attenuation of suchinterference as is picked up by the lead-in line. Third, transferimpedances are provided between the antenna and the lead-in transmissionline and between the said line and the receiver; which impedances aredesigned to increase the eil'iciency of the antenna and its lead-in lineand to permit the use of a low impedance lead-in line.

Another object of the invention is to provide a specific adaptation ofthe antenna and its associated lead-in line to a conventional multi-bandreceiver.

Other objects and features will appear from the following descriptionand the accompanying drawing. In the drawing:

Fig. 1 is a diagrammatic illustration of the antenna system;

Fig. 2 is a similar illustration of the adaptation of the antenna systemto a multi-band receiver;

y and Fig. 3 illustrates a modication.

In the specific embodiment of Fig. 1 which will serve to illustrate theantenna system, but which is not intended to limit the invention in anyway,

the antenna is divided into two parts, as illustrated, by one of theimpedance devices above mentioned. One part of the antenna may beapproximately I'l feet in length and the other part may be of anyconvenient length greater than the length of the rst part. y It has beendetermined, however, that the longer part of the antenna l shouldpreferably be about 40 feet and that there is no advantage in having thelonger part of the antenna of greater length. These two parts of theantenna may be connected respectively to the (Cl. Z50-20) terminals orextremities of a small air core coil A. The lead-in or transmission linemay comprise two conductors which are closely twisted together, orotherwise transposed, and the extremities a and b of tlse conductors atthe antenna end of the line may be connected respectively through smallcondensers C1 and C2 to points on coil A trisecting the coil or dividingit into three parts which may be about equal or preferably the centralpart may be somewhat smaller than the outer parts. Extremity a may beconnected to the coil tap nearer the longer antenna section, whileextremity b may be connected to the coil tap nearer the shorter antennasection. These same extremities of the lead-in line may also beconnected respectively through smaller condensers Ca and C4 to therespective ends or terminals of the coil A, the extremity a beingconnected in this manner to the coil terminal at the shorter antennasection, while b is connected to the coil terminal at the longer antennasection.

Considering the transfer impedance between the lead-in transmission lineand the antenna, it will be noted that there is provided an iinpedanceunit comprising a plurality of condensers and coils and that each of thecoils is closely coupled to the other coils. Due to the large number ofloop circuits, each including an inductance and a capacitance, and duefurther to the high degree of coupling existing between these severalcircuits, the unit will be at least partially resonant at a large numberof different frequencies and by proper design of the several electricalparameters, these diierent resonant frequencies may be made to cover theentire frequency range for which it is desired to use the antennasystem. As has been mentioned, the number of individual resonant pointshas been augmented by the high degree of coupling, that is byover-coupling the several inductances, which effect may be brought aboutby winding the several sections on the same form,and the number ofresonant frequencies may be further augmented by using this symmetricaldevice with a non-symmetrical antenna system. Thus, the differentcapacitances associated with each end of the unit due to the differentlengths of the antenna sections connected thereto will increase thenumber of points at which the unit is at least partially resonant.

I'he condensers Ci and C2 may be larger than the condensers C3 and C4and, consequently, will have a lower impedance. For example, theimpedances of C1 and C2 may be one-tenth those of Cs and C4. Further,the impedance of the central section of the coil A to which C1 and C:are

connected will be small as compared with the impedance of the entirecoil. Thus, looking into the unit from the transmission line, it willappear as a relatively low impedance, whereas looking 5 into the unitfrom either section of the antenna., it will appear as a relatively highimpedance.

The impedance matching between the high impedance antenna and the lowimpedance lead-in line is brought about in this fashion, the transferunit, due to its large number of resonant frequencies. serving to matchthe impedance of the antenna to that of the transmission line over awide frequency range.

The transfer impedance device connecting the l5 lead-in or transmissionline to the receiver may comprise a coil B similar to coil A but havingmore turns. The ends of this coil or points thereof near the ends may beconnected to switch terminals, and the coil may be tapped at two pointsequidistant from its ends or the said points, and these latter pointsmay also be connected to switch terminals. By suitable switch elementsS1 and Sz, the ends or terminals c and d of the lead-in line at thereceiver, corresponding respectively to extremities a and b, may beswitched at will either to the outer contacts of coil B or to theintermediate contacts so as to include either substantially the entirecoil B in the primary circuit or to include in such circuit only theportion of the coil between the innermost tapped points. The side of thecoil B associated with the line terminal c may be grounded and suitableprovision may be made for connecting that side of the coil to the usualground terminal of the receiving set, while the other side of the coilmay be connected to the usual antenna terminal of the receiver.

In using the device for the reception of the high frequency band, whichmay include the range from 1.5 to 23 megacycles, the switching elementsSi and S2 may be positioned to include the larger portion of coil B inits primary circuit. Conversely, when the device is used for receptionin the low frequency band, which may include the range from 0.5 to 1.5megacycles, the switch elements Si and S2 may be positioned to includeonly the central portion of coil B or, in other words, the portionbetween the inner coil taps, in the primary circuit.

In the preferred form of the invention, as illustrated in Fig. 2, theswitching in conjunction with the coil B is accomplished by the sameswitching devices which are customarily used to change the tuningelements of the receiver for reception of different frequency bands, thecoil B supplying signal to the transformer T. As will be apparent, thesignal obtained from the transmission line will appear across theinductance B and will be transferred to the tunable circuit comprisingthe secondary of the transformer T and the variable condenser CT, bymeans of the circuit including the I. F. trap, the condenser C5, theprimary of the transformer T, and the condenser Cs. The I. F. trapcomprises a par- 65 allel tuned circuit tuned to the intermediatefrequency of the receiver and serves to attenuate any signals ofintermediate frequency which might be picked up by the antenna. 'I'he I.F. trap will have a very high impedance to signals of the intermediatefrequency but will have a low impedance to signals of other frequencies,including the signals which it is desired to receive. The condensers C5and Cs serve as blocking condensers and present a negligibly small im-75 pedance to signals of radio frequency. Thus,

signal energy from the coil B will be transferred to the primary windingof the transformer T, and

thence to the secondary and to the input circuit of the valve V. Thesecondary of the transformer may be modified by short-circuiting varioussections of it by means of the switch X and contacts I, 2, 3 and 4. Aswill be apparent, in the position shown contacts I, 2, 3 and 4 are allconnected together and hence only that section of the secondary betweenthe grid of the valve V and the tapped point of the secondary connectedto switch point 4 is operable. In the next position of the switch,switch points I, 2 and 3 would be connected together, whereas point 4would be disconnected, and thus an additional portion of the secondaryinductance would be Included in the tunable circuit. Likewise, in athird position of the switch, contacts I and 2 would be connectedtogether but 3 and I would be free, and in the fourth position of theswitch all of the contacts would be free. The various condensers markedC7, C8, Ca and Cio, are padding condensers and serve to orientate thetuning of the circuit with that of the oscillator of the receiver, whichis not shown. Thus, as will be apparent, in the different switchpositions which adapt the receiver to select signals in differentfrequency bands, the transmission line from the antenna is connected todifferent portions of the coil B, and in each case these portions are soselected that the most eflicient transfer of signals in the desiredfrequency band to the input circuit of the valve Vis obtained.

As illustrative of a manner in which the present invention may bepracticed, the coil B may connect the antenna to ground so that the coilwill by-pass disturbances of relatively low frequency compared with thesignal frequency. In other words, the circuit to ground through the coilB offers low impedance to low frequency cur-l rents or surges and thuseffectively prevents disturbances from entering the receiver proper.Thus, in addition to its function as a transfer impedance device duringmulti-band reception, the coil B may serve the purpose above noted whenthe ordinary antenna is used during single band reception. Moreover, thecoil B presents high impedance to the relatively high frequency signalcurrents permitting them to enter the receiver.

It will be noted also that in the system of FIg. 2, the resistor R maybe supplied with unidirectional current from a suitable biasing sourceto bias the stage V and thus control the amplification. For example,automatic volume control may be incorporated by connecting resistor R toa variable biasing source in accordance with conventional methods. Itwill be seen that the condensers C5 and C6 will each present infiniteimpedance to the unidirectional current, thus preventing the by-passingof such current to ground. Thus, the condenser C5, the transformer T,and the condenser C5 are included in a series circuit between theantenna terminal and ground, and these elements in cooperation with coilB serve the above purposes.

Connections may be made from the coil B to switch points or contacts onthe sv. itching devices which are used to effect change of the tuningelements as above mentioned. The switching devices will, of course, beespecially adapted for this purpose. It is merely necessary, however,

to provide on those devices properly located selectable contacts whichare engageable by a contact properly located on the movable part of theswitch. A pair of terminals c' and d may be provided with conductorsrunning from these terminals to the switching devices, the arrangementbeing such that these conductors may be switched to different points oncoil B in the same manner that the lead-in line terminals c and d areswitched in the device of Fig. 1. Two of the said switching devices maybe used for this purpose, one for switching each of the said conductors.The conductor for terminal c' may be associated with the ground end ofcoil B.

In the device of Fig. 2, band selector switches X and Y are employed,which switches are preferably adapted for operation simultaneously witheach other and with other Idevices by a conventional gang controlarrangement, as indicated by the broken line representation. The switchX controls the conventional multi-band .transformer T which suppliessignals to the first vacuum tube stage V. The switch X has fourpositions designated 1, 2, 3 and 4. In the illustration, the switch isshown in position 4. Each of these positions may correspond to thefollowing wave bands:

1. 0.5- 1.5 megacycles.

2. 1.5- 4.5 megacycles 3. 4.1-10.0 megacycles 4, l0.0-22.5 megacyclesSwitch X is also provided with contacts t, r, and s while switch Yhascorresponding contacts p, o and n, these contacts being connected asillustrated to terminals c and d and to coil B. Switch Y may also have four positions, corresponding to those of switch X, which may be utilizedif desired to effect switching of the oscillator. This may be done bymaking the proper electrical connections and by providing a suitablemovable contact in switch Y to effect the desired switching operation.

In positions 2, 3 and 4 of the switches X and Y, the contact o isconnected to the contact n and the contact ris connected to the contacts. In position 1 of the switches, corresponding to the long wave band,the contact o is connected to contact p, while contact r is connected tocontact t. These switching operations properly adapt the coil B forreception of high or low frequency bands when the antenna and lead-inline of Fig. 1 are used. The switching is accomplished simultaneouslyvwith the switching of the tuning elements of the receiver.

With such arrangement, the receiver may be used in conjunction witheither an ordinary antenna or with the improved antenna of Fig. 1.

If the ordinary antenna is used, it will, of course,

be connected to the antenna terminal of the receiver and no connectionwill be made to the two terminals c and d. In such case, the switchingelements provided in conjunction with coil B will have no effect, thesaid coil cooperating with elements C5, Ce, T and R in the manner abovedescribed. If, however, it is desired to use the improved antenna systemof Fig. 1, the lead-in line terminals c and d thereof may be connectedto the terminals c yand d' of Fig. 2, terminal c of 4the antenna lead-inline being connected to terminal c' and terminal d of the antenna leadinline being connected to terminal d. In such case, the switching deviceswill function as described above when the receiver is operated forreception of different frequency bands. In either case, the groundterminal of the receiver should be connected to any convenient groundsuch as a. water pipe, etc. in the usual way.

In the simplified modification of the device shown in Fig. 3, one of theswitching operations may be eliminated by permanently connecting theconductor from terminal c to the grounded end of coil B and by switchingthe conductor from terminal d between the other end of the coil and anintermediate point thereof. In such case, the tap connection of coil Bmay be located so that there is the same ratio of turns of the collbetween high and low frequency operation as there is in the antennasystem of Fig. 1 and in the system of Fig. 2. This modification reducesthe cost of manufacture, inasmuch as it requires special adaptation ofonly one of the switching devices X and Y.

In operation of the system at high frequency, the short side only of theantenna appears to be effective. Experiment has indicated that anantenne, length of 17 feet is best adapted for reception through thehigh frequency or short-wave band. The impedance device rst describedabove serves to match the high impedance antenna to the relatively lowimpedance lead-in line. As explained above, the coil and condensersprovide a plurality of resonant circuits having different naturalperiods so that the impedance device as a whole does not have a markedresonant frequency, thus making the gain as nearly uniformat allfrequencies as possible and causing the response curve to be more even.At the receiver end of the lead-in line, the coil-B acts as a returncircuit for a reason which will appear later.

At low frequency, the longer side of the antenna is effective and theimpedance device at the antenna again serves to match the impedances ofthe antenna and the lead-in line. At the receiver end Iof the lead-inline, the coil B matches the relatively low impedance transmission lineto the antenna coil of the receiver which, in this case, is ofrelatively high impedance.

It will be seen that in the reception of both high and low frequencywaves, there is obtained a gain in signal strength due to the moreefficient transfer of energy from the antenna to the receiver by virtueof the impedance matching. In either case also, interference caused byelectrical disturbances will be reduced as fo1lows:-First, the use oftwo conductors closely twisted together serves to reduce the effect ofdisturbances by reducing the possible mutual inductive or capacitivecoupling between the lead-in line and the source of disturbances.Second, a disturbance set up in a given direction in one conductor ofthe line will be induced also in the other conductor and will have thesame direction and magnitude. Inasmuch as the two components of thedisturbance will oppose each other in the lead-in circuit, they willlargely counteract or neutralize each other at the coils constitutingthe terminals of the circuit. Third, the surge lmpedance of the lowimpedance lead-in line has been chosen so as to give a smooth responsefor signal energy but to reduce to a minimum the pick-up of disturbanceenergy.

In the specific example of the above-described device, the coil A maycomprise 108 turns of #30 SCE wire wound in a honeycomb coil on a 1/2"bobbin and tapped at the 36th and 72nd turn. Coil B may comprise 207turns of #32 SCE wire wound in similar fashion and tapped at the 92ndand 115th turn. Condensers C1 and C2 may each have a value of 1650micro-microfarads, while condensers C3 and C4 may each have a value of130 micro-microfarads. The transmission line may have a surge impedanceof about 100 ohms at 1000 cycles. Such a line is of sumciently lowimpedance to be effective and yet is not low enough to be expensive, itbeing noted that the cost varies approximately inversely with theimpedance.

It will be understood, of course, that the invention is capable ofmodication and that the present disclosure is not intended to limit theinvention.

I claim:

l. An antenna system for radio receivers, comprising an antennaterminal, a condenser, a first impedance connected to said condenser, aground terminal, a second impedance connected to said ground terminal,said second impedance being larger than said first impedance at lowfrequencies but less than said first impedance at high frequencies, saidcondenser and said impedances forming a series circuit between saidterminals, a vacuum tube having input elements, means for transferringhigh frequency signals appearing across said first impedance to saidinput elements and for conductively connecting said elements to theterminals of said second impedance. and a third impedance element of lowimpedance at low frequencies and of high impedance at high frequenciesconnected to said antenna terminal and said ground terminal.

2. An antenna system for radio receivers, comprising an antennaterminal, a condenser, a first impedance connected to said condenser, aground terminal, a second impedance connected to said ground terminal,said second impedance being larger than said first impedance at lowfrequencies but less than said first impedance at high frequencies, saidcondenser and said impedances forming a series circuit between saidterminals, a vacuum tube having input elements, means for transferringhigh frequency signals appearing across said first impedance to saidinput elements and for conductively connecting said elements to theterminals of said second impedance, and a third impedance element of lowimpedance at low frequencies and of high impedance at high frequenciesconnected to said antenna terminal and said ground terminal, said thirdimpedance being provided with terminals for connection to a transmissionline.

3. In a multi-band radio receiving system, an antenna, a transmissionline, coupling means between said antenna and said transmission line fortransferring signals in a plurality of frequency bands from said antennato said transmission line, an amplifier including a space dischargedevice and a tunable input circuit for said device, means fortransferring signals from said transmission line to said input circuit,switching means for modifying said input circuit to select signals inany one of said plurality of frequency bands, and switching meansoperable simultaneously with said first switching means for modifyingsaid signal-transfer means to transfer signals efficiently in theselected frequency band.

4. In a multi-band radio receiving system, an antenna, a. low impedancetransmission line, coupling means between said antenna and saidtransmission line for transferring signals in a plurality of frequencybands from said antenna to .said transmission line and for matching theimpedances of said antenna and said transmission line, an amplifierincluding a space discharge device and a tunable input circuit for saiddevice, means for transferring signals from said transmission line tosaid input circuit and for matching the impedances of said transmissionline and said input circuit, switching means for modifying said inputcircuit to select signals in any one of said plurality of frequencybands, and switching means operable simultaneously with said firstswitching means for modifying said signal-transfer means to transfersignals efficiently in the selected frequency band.

5. In a multi-band radio receiving system, an antenna, a two conductortransmission line, coupling means between said antenna and saidtransmission line for transferring signals in a plurality of frequencybands from said antenna to said transmission line, an amplifierincluding a space discharge device and a tunable input circuit for saiddevice, means for transferring signals from said transmission line tosaid input circuit, switching means for modifying said input circuit toselect signals in any one of said plurality of frequency bands, andswitching means associated with each conductor of said transmission lineand operable simultaneously with said first switching means formodifying the connection of said line to said signal-transfer means toeffect efficient transfer of signals in the selected frequency band.

6. In a multi-band radio receiving system, an antenna, a transmissionline, coupling means between said antenna and said transmission line fortransferring signals in a plurality of frequency bands from said antennato said transmission line, an amplifier including a space dischargedevice and a tunable input circuit for said device, means fortransferring signals from said transmission line to said input circuit,said signal transfer means comprising a coil having tapped points,switching means for modifying said input circuit to select signals inany one of said plurality of frequency bands, and switching meansoperable simultaneously with said first switching means for connectingsaid transmission line to different tapped points of said coil to effectefficient transfer of signals in the selected frequency band.

DAVID P. EARNSHAW.

